1. Field of the Invention
The present invention relates generally to an electrographic apparatus and more specifically to an improved structural arrangement in an electrographic apparatus of the type having a densitometer, which arrangement achieves improved measuring of marking particle density on a photoreceptor or the like.
2. Description of the Prior Art
It is known in the copying art to use light sensors to measure the density of a powderous substance or the like. The following patents are examples of similar devices, which are herein incorporated by reference where appropriate for appropriate teaching of additional or alternative details, features, and/or technical background. It is noted that the following descriptions are reliant upon excerpts from the identified patents, thus any inaccuracies in the following descriptions are a result of the patent's representation. For example, U.S. Pat. No. 4,806,002 discloses a densitometric sensing device in which a density measuring head, which is part of the device, traverses an ink test strip printed on a page which is fed to or placed on the device. The problem of misalignment between the placed ink test strip and the scanning direction of the density measuring head is avoided by providing a plurality of density measuring receivers in the measuring head, mounted transverse to the direction of travel so that the ink test strip is always under some of the receivers. The measured density values of all of the receivers are passed to a comparison circuit which cooperates with a logic circuit to determine from the values of the signals themselves which should be used to produce a composite density value, which is then used as the density for the zone being scanned.
U.S. Pat. No. 4,801,980, discloses the idea of using a toner density control apparatus constructed so that the toner density is controlled on the basis of the density. The density is detected by a sensor, of a patch image obtained by developing an image on a reference density plate on the surface of a photosensitive drum, characterized in that the apparatus includes means for controlling the rate of emission of a light from a light-emitting element in the sensor so that an output from a light receiving element, which is adapted to receive a light reflected on the surface of a non-image forming region of the photosensitive drum in the sensor, is in a predetermined level.
U.S. Pat. No. 4,551,004, discloses an apparatus for monitoring toner concentration on a photoreceptor surface. The apparatus includes a light emitting diode (LED), a phototransistor, a beam splitter and a lens disposed between the beam splitter and the photoreceptor surface to collimate the light beam between the lens and the photoreceptor surface. A portion of the light emitted from the LED is transmitted through the beam splitter and the lens to the photoreceptor surface. Collimated light is reflected from the photoreceptor surface back through the lens and reflected from the beam splitter to the phototransistor. The output signal from the phototransistor, because of the reflected collimated light beam, is not dependent upon the distance of the lens to the photoreceptor surface. Alternately, a second lens is disposed between the beam splitter and the phototransistor to enhance the overall resolution of the system.
Another example is U.S. Pat. No. 4,502,778, which discloses digital circuitry and microprocessor techniques to monitor the quality of toner operations in a copier and take appropriate corrective action based upon the monitoring results. Patch sensing is used. Reflectivity signals from the patch and from a clean photoconductor are analog-to-digital converted and a plurality of these signals taken over discrete time periods of a sample are stored. The stored signals are averaged for use in determining appropriate toner replenishment responses and/or machine failure indicators and controls.
U.S. Pat. No. 4,462,680 discloses a toner density control apparatus which assures always the optimum toner supply and good development with toner, irrespective of the kind or original to be copied and/or the number of copies to be continuously made. The apparatus has a detector for detecting the density of toner. The quantity of toner supply is controlled using a value variable at a changing rate different from the changing rate of the density difference between the reference toner density and the detected toner density.
U.S. Pat. No. 4,226,541 discloses illuminating a small area of a surface to be reflectively scanned, detecting the intensity of the light reflected from the small area and generating a first signal proportional thereto, detecting the intensity of the light reflected from an area at least partially surrounding the small area and generating a second signal proportional thereto, subtracting at least a fraction of the second signal from the first signal to produce a compensated signal which represents the reflectivity of the small area as compensated for the effects of scattered light, and either using the compensated signal directly as analog data or converting it to a digital output signal having a first state when the compensated signal is above a predetermined threshold and having a second state when the compensated signal is below that threshold.
In U.S. Pat. No. 4,178,095, a means exists to check for an abnormally low reflectance photoconductor during a test cycle in an electrophotographic copier machine by utilizing reflectivity sensing devices and circuitry ordinarily used for quality control where the reference signal is sensed from an area of the photoconductor normally used for document reproductions. This same means is also used for partially indicating correct circuit operation during machine operating periods when the circuit is not used for quality checking by a forced output condition not indicative of a quality check. This patent also discloses a means for producing a first signal which is a function of the reflectivity of the developer mixture, and means responsive to the number of replacement containers received by an interface means for producing an offset signal representing changes in developer mixture reflectivity caused by scanning of the carrier particles. Computation means such as a digital computer respond to the first signal and the offset signal to provide an accurate indication of the concentration of toner in the developer mixture.
U.S. Pat. No. 3,876,106 discloses an apparatus for continuously monitoring the concentration of toner in an electrographic developer mixture of toner and carrier particles at a magnetic brush development station by sensing the reflectivity of such mixture. The apparatus includes a radiation source for illuminating the mixture, first and second photocells for producing analog signals representative of the reflectivity of the mixture and the intensity of the radiation source, respectively. The apparatus further includes digital processing apparatus having an analog to digital converter and a programmable digital computer, having a stored program, which in response to the analog signals, produces in accordance with such stored program a representation of the relative proportion of toner particles in the mixture.
U.S. Pat. No. 3,777,173 discloses an apparatus for measuring the toner concentration in the developer of a xerographic copying apparatus. The measuring apparatus includes a toner collecting plate, and supply means for providing a stream of the developer containing the toner whose concentration is to be measured. The developer stream is directed onto a surface of the collecting plate to cause toner particles to be dislodged from the carrier beads, pellets or granules to which the toner particles are electroscopically adherent to deposit a layer of toner on at least a portion of the collecting plate. The developer stream removes toner from the collecting plate when the concentration of toner in the stream decreases. Means is provided for sensing the amount of toner deposited in the form of the aforementioned layer.
Lastly, U.S. Pat. No. 3,981,272 discloses the use of a sensor that senses that the toner concentration of the developer is below a predetermined set point level.
The ideal goal in xerography is to have the correct amount of toner deposited onto the photoreceptor on a continuous bases. Too little toner causes poor copy quality, too much toner causes excess wast and increased expense to run the system. Machines that can achieve this balance will have a tremendous competitive edge. Thus far, the use of a densitometer has been employed. However, the current densitometers either only work for black and white images, and not color, or are too simplistic in their ability to achieve higher resolution of the particles upon the photoreceptor or the like.
Therefore, in response to the problem, a need exists for a way to provide for both black and a high resolution color densitometer in a printer or copier.
As a result, the present invention provides a solution to the described problems and other problems, and also offers other advantages over the prior art.